Focus on the aerodynamic features of the underbody has gradually increased over the past years also for registered cars (i.e. cars which can circulate freely on roads open to the public). Initially, the trend was to make cars simply with fully flat underbodies with the sole objective of reducing aerodynamic drag. Later on, underbodies were shaped to reduce aerodynamic drag and also to increase downforce, i.e. so that the action of air (at high speeds) can generate a downward thrust on the car which increases road hold.
In order to increase downforce, it has been suggested to use an aerodynamic diffuser (or aerodynamic extractor) arranged in rear position, and comprising a chute, which constitutes the underbody of the car, faces the road surface, and has a height from the road surface which progressively increases when longitudinally moving from the front to the rear. At high speed, the air flowing underneath the car (i.e. between the underbody of the car and the road surface) crosses the aerodynamic diffuser and progressively increases its volume (as mentioned above, the aerodynamic diffuser has a height from the road surface which progressively increases when longitudinally moving from the front to the rear) and thus tends to gradually reduce its pressure, generating a vacuum under the vehicle as a consequence; such a vacuum creates downforce, i.e. an aerodynamic thrust which pushes the car downwards.
However, the aerodynamic development of the underbody of a car is subject to numerous geometry constraints deriving from the presence of dimensions which cannot be eliminated, such as the passenger compartment, the engine (particularly in the case of a mid or rear engine arrangement), the fuel tank, the exhaust system and the frame structure. These constraints considerably limit the extension of the shaped surfaces of the underbody intended to create downforce, and consequently in the known cars the underbody is completely flat to the rear part of the car (beyond the rear wheel axle) where the development of an aerodynamic diffuser starts. Consequently, in the known cars, the performance of the aerodynamic diffuser arranged on the underbody in rear position is adequate but not excellent (in particular if compared to the aerodynamic diffusers used on racing cars).
Patent application DE4102073A1 describes a car provided with an underbody which delimits the lower side of the car, faces the road surface and comprises an aerodynamic diffuser which is developed in rear position and has a chute, having a height from the road itself which progressively increases when longitudinally moving from the front to the back; the underbody also comprises a pair of aerodynamic channels, each of which extends longitudinally, leads to a radiator of a “V”-engine arranged in rear position, is open at the bottom towards the road surface, and has a transverse section with an area which progressively increases when longitudinally moving from the front to the rear.